It is known practice in the design of a geared differential mechanism for driving axles of a wheeled vehicle to provide a limited slip characteristic using friction disks to establish a controlled torque bias as driving torque is delivered from a driveshaft to the axle shafts, which in turn are connected to the traction wheels of the vehicle.
The differential gearing in conventional differentials includes a crown gear or ring gear that is connected drivably to a drive pinion. A pair of differential side gears is situated within a differential housing, one side gear being connected to one axle half shaft and the other being connected to the other axle half shaft. Differential pinions continuously engage the side gears. The pinions are mounted so that they rotate with the differential carrier housing.
In the case of a limited slip differential, the pinion shafts are mounted between two pressure rings that are splined to the differential housing. The pressure rings cannot rotate relative to the housing, but they can shift axially as torque is transmitted to the pinion shaft. The pinion shaft engages ramp surfaces on the pressure rings so that torque applied to the differential housing will be translated into an axial force on the pressure rings.
A friction clutch is situated on each side of the differential mechanism. At least one friction disk of each clutch is connected to the differential housing and at least one adjacent friction disk of each clutch is connected drivably to a separate one of the side gears. As torque is transmitted through the differential mechanism, an axial force acting on the pressure rings engages frictionally the clutch disks, thereby providing a torque bias which resists motion of the side gears relative to the housing.
The amount of the torque bias that is created in this fashion is directly related to the torque being transmitted through the engaged drive pinion and ring gear and to the geometry of the pressure rings (i.e., the ramp angles of the ramp surfaces). Similarly, the torque transmitting capacity of the clutches is directly proportional to the torque being delivered to the axial shafts.
Wheel spin is restricted because of the torque bias developed by the axial force components acting on the pressure rings. It is possible, therefore, for the vehicle to accelerate even if one traction wheel is on a low friction surface. The torque bias further reduces the possibility of skidding due to a yaw torque if the vehicle encounters a low friction surface or severe bumps.
During cornering of a vehicle equipped with a torque-sensitive limited slip differential, the innermost traction wheel maintains traction as load is transferred to the outside traction wheel of the vehicle. The torque transferred to the outermost traction wheel equals the torque developed at the innermost traction wheel multiplied by the bias ratio. This improves the steering response and reduces the possibility of understeering.
It is also known design practice to complement the driving torque-induced axial forces acting on the friction clutches with a preload spring force. This tailors the bias torque ratio to suit particular drivability requirements. The initial bias at the breakaway torque is determined by spring load.
Another design approach involves the use of a hydrostatic speed responsive torque bias. Examples of a hydrostatic limited slip differential mechanism of this kind may be seen by referring to U.S. Pat. Nos. 5,595,214, 5,611,746, and 5,536,215. Each of these patents is assigned to the assignee of the present invention.
The hydrostatic torque bias differential mechanism of the prior art patent references mentioned above is accomplished by integrating a Gerotor pump with the differential gearing of the differential mechanism. The Gerotor pump has a first pumping gear member with internal gear teeth, which register with a companion gear member with external gear teeth. The two gear members are eccentrically mounted, one with respect to the other. The internal gear member of the hydrostatic Gerotor pump has one fewer internal teeth than the number of external teeth of the companion gear member.
The Gerotor pump develops a pumping chamber between the internal and external Gerotor pump teeth, the volume of the pumping chamber being a maximum when the Gerotor pump elements are positioned to provide maximum communication with a fluid inlet port. A fluid discharge port is angularly spaced from the inlet port. As the pumping chamber decreases in volume, the communication between the inlet port and the pumping chamber is progressively interrupted as communication between the pumping chamber and the outlet port progressively increases. Fluid is circulated through the Gerotor pump when one side gear of the differential gearing rotates relative to the differential housing.
Provision is made in the hydrostatic fluid flow circuit for a controlled restriction in the fluid flow path. The energy that is developed by the pumping action of the Gerotor pump members increases as the relative speed of the differential side gear with respect to the differential housing increases.
Because the Gerotor pump is a positive displacement pump, the torque bias developed by the pump is proportional to the relative speeds of the pumping members regardless of the magnitude of the torque being transmitted through the differential.